ramips: add support for Xiaomi Mi Wi-Fi Router 3G v2

ramips: add support for Xiaomi Mi Wi-Fi Router 3G v2

- CMIIT ID: 2019AP2581
- SoC:      MediaTek MT7621
- Flash:    16MiB NOR SPI (GigaDevice GD25Q128B)
- RAM:      128MiB DDR3 (ESMT M15T1G1664A)
- Serial:   As marked on PCB, 3V3 logic, baudrate is 115200, 8n1
- Ethernet: 3x 10/100/1000 Mbps (switched, 2xLAN + WAN)
- WIFI0:    MT7603E 2.4GHz 802.11b/g/n
- WIFI1:    MT7612E 5GHz 802.11ac
- Antennas: 4x external (2 per radio), non-detachable
- LEDs:     Programmable "power" LED (two-coloured, yellow/blue)
            Non-programmable "internet" LED (shows WAN activity)
- Buttons:  Reset

INSTALLATION:

Bootloader won't accept any serial input unless "boot_wait" u-boot
environment variable is changed to "on". Vendor firmware (looks like
an illegal OpenWrt fork) won't accept any serial input unless
"uart_en" is set to "1". Tricks to force u-boot to use default
environment do not help as it's restricted in the same way.

With bootloader unlocked the easiest way would be to TFTP the
sysupgrade image or to sysupgrade after loading an initramfs one.

For porting the flash contents were changed externally with an SPI
programmer (after lifting Vcc flash IC pin away from the PCB).

Forum thread [0] indicates that this device is identical to "Xiaomi Mi
Router 4A Gigabit Edition".

[0] https://forum.openwrt.org/t/xiaomi-mi-router-4a-gigabit-edition-r4ag-r4a-gigabit-fully-supported-but-requires-overwriting-spi-flash-with-programmer/36685

Signed-off-by: Paul Fertser <fercerpav@gmail.com>